Manhattan U.S. Attorney Preet Bhararas office is seeking information from businesses dealing in bitcoin on how some of them handled cyber-attacks that hamstrung several exchanges in recent weeks, a source familiar with the probe said on Wednesday. Subpoenas have been sent to numerous businesses, including Mt. Gox, once the largest bitcoin exchange, as well as other firms that did business with the Tokyo-based company, the source said. Prosecutors want to know more about the nature of the cyber-attacks on Mt. Gox and other exchanges and how those exchanges dealt with them.